A checkpoint iterates over every dirty block in the system as of a point in time, writing them out to disk. Then, that information is flushed to permanent storage, via the same mechanisms WAL writes are. Once that's done, if your system crashes, recovery from the crash will start from this new last point where the database was sure that everything on disk was consistent. In this way, the database constantly takes forward the recovery-starting position to the point in time the previous checkpoint started at. Everything that happened to dirty blocks before this is guaranteed to have been written to the database blocks on disk, and therefore the amount of potential information that might be lost in a crash is kept ...
Checkpoint processing basics
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